This invention relates to replacement fuel assembly alignment pins for installation in the upper core plate of a nuclear reactor, working exclusively from the bottom side of the upper core plate. The invention provides a replacement pin structure and method for mounting the replacement pin from below the core plate, which captures the remnants of the existing fuel alignment pin and nut, thus eliminating the risk of internal damage as well as the cost associated with accessing the top of the upper core plate when replacing alignment pins.
Pressurized water nuclear reactors have fuel assemblies supported in the reactor core between upper and lower core plates. The lower core plate is supported by a core barrel, which surrounds the reactor core between the upper and lower core plates. A number of fuel assemblies reside between the upper and lower core plates. Each fuel assembly has an upper end structure known as the top nozzle, and a lower end structure or bottom nozzle. The upper nozzle comprises an upper hold-down plate and a lower adapter plate, rigidly coupled by sidewalls forming a top enclosure on the fuel assembly. The upper and lower nozzles are arranged parallel to one another and parallel to intermediate grid plates, the grid plates having openings through which a plurality of spaced parallel fuel rods extend between the nozzles. Extending between the nozzles and through the grid plates, and perpendicular thereto, are guide tubes or thimbles which support movable control rods. The nozzles, thimbles and grid plates form an integral fuel assembly unit for carrying, for example, about three hundred fuel rods as a unitary structure, and a plurality of such assemblies are placed in close proximity between the upper and lower core plates of the reactor core.
The control rods are moved axially into the fuel assembly to damp the nuclear flux, and out of the fuel assembly to increase nuclear flux, namely by either absorbing or allowing passage of the products of nuclear fission passing between the fuel rods. In this manner, it is possible to control the generation of heat in the reactor core, which is submerged in a heat transfer fluid, i.e., pressurized water.
The control rods are controllably lowered vertically through the upper core plate into thimbles of the fuel assemblies. Therefore, it is important that the fuel assemblies be accurately aligned relative to the core plates in order to allow accurate and free passage of the control rods in the fuel assembly thimbles. The control rods are mounted relative to the bottom flange of a guide structure which includes a guide tube support pin engaging in the upper surface of the core plate and thus referencing the control rods to the core plate. For likewise referencing the fuel assembly to the core plate, the core plate has protruding alignment pins oriented parallel to the axes of the fuel rods, control rods and thimbles. The alignment pins attached to the core plates mate with openings in the nozzles of the fuel assemblies, thereby fixing the relative positions of the fuel assembly thimbles and the control rods.
The fuel assemblies must be serviced periodically, for example for replacement of the fuel rods, and can be removed from engagement with the core plates for this purpose, and thereafter replaced. The alignment pins are sometimes damaged in such operations. The failure of particular alignment pins can adversely affect the ability to control the reactor core, due to resulting misalignment of the control rods and the thimbles for receiving them. Therefore, the alignment pins are routinely checked, and sometimes replaced.
The attachment of the alignment pins to the core plate is also necessarily durable and rigid, to keep the alignment pins accurately fixed in position as well as to minimize damage. In U.S. Pat. No. 4,820,479--Hornak et al, an alignment pin is disclosed with a conical nose for engaging a fuel assembly, a shoulder resting against the lower surface of the upper core plate, and a shaft protruding through the upper core plate. The shaft of the alignment pin is threaded to receive a nut, for clamping the alignment pin to the upper core plate between the shoulder and the nut. The shaft also has a transverse opening which resides above the clamping nut when tightened down. A deformable locking cup is placed on the shaft of the alignment pin over the clamping nut, engaged to the clamping nut preferably by welding, and crimped to engage in the transverse opening in the shaft of the alignment pin. The locking cup thus cannot turn relative to the shaft and likewise prevents rotation of the clamping nut.
It is also known in such an arrangement to weld the clamping nut directly to the top surface of the upper core plate and thereby prevent the clamping nut from loosening. The Hornak alignment pin mounting is considered advantageous because it is more readily replaceable. To remove an alignment pin, the deformable locking cup is forced out of engagement with the transverse opening in the shaft of the pin, and the clamping nut can be loosened. However, this requires access to the top surface of the upper core plate to loosen and remove the clamping nut, and access to the bottom surface of the upper core plate to hold and/or retrieve the alignment pin.
Other forms of clamping nuts on the top surface of the upper core plate are also known. The clamping nut can be disposed in a counterbore in the upper core plate, fixed to the core plate by various forms of locking pins, screws or keys, etc. A more permanent and secure coupling of the clamping nut to the upper core plate is desirable to obtain a durable mounting for the alignment pin. A more durable mounting is of course more difficult to remove than a less durable one, and in either case, removing the mounting requires access to the top of the upper core plate.
Alignment pins are replaced using a remotely controlled and/or at least partly robotic techniques, monitored by video, to avoid exposure of plant personnel to the radioactive core elements. A certain amount of space is needed to accommodate the tools and viewers needed to conduct pin replacement operations. Typically, part of the upper internals of the reactor, in the region over the upper core plate, must be removed to gain access to the clamping nuts of the alignment pins. Removal of such structures, operations in this region, and replacement of the structures, are time consuming and may cause damage to the structures in the region. For all these reasons, it is desirable to enable a pin replacement technique which is conducted entirely from below the upper core plate.